Servo-control unit for a variable prestressed load

ABSTRACT

The invention relates a servo-control unit for a load ( 2 ) driven in rotation by motorization means ( 1 ) connected to the Load by two reduction gears ( 3, 4 ) and comprising several sensors for measuring their movements It includes a prestressing motoring system ( 5 ) arranged between the two reduction gears ( 3, 4 ) so as to apply a positive and respectively a negative torque to each of the reduction gears. The prestressing system ( 5 ) is designed to apply a torque at any particular point of the operating field of the reduction gears and to introduce therein a variable prestressing level. The prestressing system ( 5 ) is a jack ( 14 ) driven by a motor ( 17 ) and co-operating with a rack or a screw-nut system ( 15 ). The invention is useful for motorization of a turret mounted on a vehicle.

BACKGROUND OF THE INVENTION

The technical scope of the present invention is that of devices to drivea load with reduced or no backlash between the motor and the load.

When a load is driven in movement, and notably in rotation, the backlashbetween said load and the drive motor must be reduced to a minimum so asto obtain the precise positioning of the load with respect to a givenreference. This problem is of extreme importance within the scope of thelaying of a weapon mounted in the turret of an armed vehicle.

Various principles to eliminate any backlash are known and used invarious branches of the technique.

By way of illustration, patent FR-2221981 proposes two independent andslightly opposing chains to eliminate the problems of backlash, butusing two motors. This is the main drawback to this device.

Patent FR-2538136 proposes to release the servo-driven chain from theelasticity of the transmission so that the motorisation is comparable tothat of a torque motor connected directly to the load. In the proposedsystem, the motorisation is constituted by a motor connected to the loadby two reduction gears whose elasticity is employed to eliminate thebacklash in a transmission by reduced prestressing. This reducedprestressing is enough to absorb the different homokinetic type errorsthat may appear between motor and load. This system is perfectly welladapted when the ratio between the elasticity of the reduction gears andthe maximal homokinetism error is high.

However, when the harmonisation errors exceed around 10% of theclearance between the motor and the load required to transmit themaximum motorisation torque, the principle of approximately fixedprestressed load is used to avoid the arcing of two chains set inparallel, locally or globally over the full kinematics. The inventionaims to supply a device having this property.

BRIEF SUMMARY OF THE INVENTION

The invention thus relates to a servo-control unit for a load driven inrotation by means of motorisation connected to the load by two reductiongears and comprising sensors for measuring their movements, wherein itcomprises a prestressing motoring system arranged between the tworeduction gears so as to respectively apply a positive and a negativetorque to each of the reduction gear.

Advantageously, the prestressing system is designed to firstly apply toa torque at any point of the operating field of the reduction gears andsecondly to introduce therein a variable prestressing level.

According to a first embodiment, the prestressing system is a jackdriven by a motor and co-operating with a rack or a screw-nut system.

Advantageously, the prestressing system comprises a power stage boxhaving an epicyclic gear train.

According to another embodiment, the device comprises prestressinginputs on the crown of the reduction gear with prestressing motorisationprovided by hydraulic jack and pressure servo.

According to yet another embodiment, the prestressing system is a doublereduction chain assembly integrating a jack and at least one inputpinion whose axis of rotation has a degree of freedom with respect tosaid assembly.

Advantageously, the pinion is connected to the output of said assemblyby an Oldham coupling type system.

Advantageously again, the device comprises a connecting rod hinged so asto ensure the radial displacement of the pinion.

According to yet another embodiment, one of the reduction gears has aslant toothed gear stage.

Advantageously, the pinion of the stage has an axial degree of freedomwith respect to the wheel, said degree of freedom being used to modifythe prestressing of the two chains by a back-geared motor.

According to yet another embodiment, the prestressing means comprise ajack allowing the reduction gears to be applied to the load.

Advantageously, the prestressing motorisation system incorporateselectrical control means.

Additionally, the principle of dual motorisation may be applied, bothchains of which are respectively used to supply the positive torque andnegative torque to be transmitted to the load. In this case, theprestressing is made actively by specific control of the motorisationsof each of the chains, thereby ensuring the transmission of positive ornegative torques to the load without having to pass through the backlashof the transmission chains.

The invention applies in particular to systems having a lot of backlashbetween the motor and the load, such backlash being non negligible inrelation to the angular strain of the transmission under the maximumstress to be transmitted. The second particularity lies in that thesystem is particularly sensitive to friction torques in the transmissionboth from a final accuracy but also an energetic point of view.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics, particulars and advantages of the invention willbecome more apparent from the following description, given by way ofillustration and in relation to the appended drawings, in which:

FIG. 1 is a general schematic illustration of the device according tothe invention,

FIG. 2 is a general block diagram of the device according to theinvention,

FIG. 3 shows a first embodiment of the invention,

FIG. 4 shows an epicyclical train,

FIG. 5 shows a second embodiment of the invention,

FIG. 6 shows a third embodiment of the invention, and

FIG. 7 is a section view of a fourth embodiment of the invention.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 shows that the device according to the invention schematicallycomprises a motor 1 dimensioned so as to supply a load 2 with torque andvelocity. Between the motor 1 and the load 2 a double reduction chain isplaced which comprises reduction gears 3 and 4 to connect the load tothe motor. Prestressing motorisation means 5 are placed between thereduction gears 3 and 4, each provided with an input in a position usedby the prestressing motorisation means. Naturally, this control allows adesired level of prestressing to be introduction. Lastly, a specificmeans 6 allow the controls made by the means 5 to be defined.

The structure thus described allows the reduction chains to be adaptedto the different natural anomalies of the transmission system, whichwith zero torque transmitted in each of the chains, corresponds to therelative position between the motor 1 and the load 2 in which may beincluded:

-   -   the influence of the temperature variation on the differential        dilations between the gear housing and the internal mechanical        elements.    -   the ovalization errors of a toothed or circular sector,    -   the homokinetism errors between the motor and the load, due to        errors in the tooth formation of the gears.

According to the mechanical reduction principle used, the system to addthe prestressing may take various forms. These will be classified intotwo families respectively corresponding:

-   -   to all the electromechanical systems allowing an additional        angular displacement to be added in each of the chains. The fact        in particular that there is little clearance to be added is        exploited. For example, this limited displacement may be        obtained by a radial relative displacement between the output        pinions and the sector or circular gear to be driven. The        potential degree of freedom of a reduction stage by epicyclic        train may also be used to introduce the required variable.        Another variant lies in the use of the axial displacement of a        slant toothed reduction pinion to recover the required degree of        freedom,    -   to using a hydraulic jack type system with pressure servo, which        constitutes a well adapted means to ensure the contact, of both        chains with no backlash and with controlled arcing.

The nominal advantage of the variable prestressing motorisation systemlies in that it constitutes a device that firstly compensates for thedifferential homokinetic variations between the two reduction chains andsecondly increases the prestressing of the torque to be transmittedduring transmission. This optimisation of the prestressing torque infact allows the friction torque in the motorisation chain to beminimised and thus its corresponding effect firstly upon the accuracy ofthe load's servo system and secondly on the energetic cost of themotorisation control. In fact, fixed prestressing systems allowing anapproximately constant rigidity of the motorisation chain to be ensuredin all the range of torques to be transmitted and in that of thepositions of the load directly result in the increase in the level ofthe friction torque to be overcome with the dual disadvantage on theaccuracy of the load's servo system and on the energetic cost of thecontrol.

The means proposed to vary the prestressing, using the control, in themotorisation chain must therefore be associated with adapted controls.In these controls will be differentiated firstly the principle allowingthe instantaneous torque to be applied in the prestressing chain to beassociated with the control principle allowing this prestressing to bemade on the secondary chain.

The processing to be made on the signal to be picked up from the mainchain is a conventional processing allowing a prestressing torque valuecorresponding to an absolute value of the maximum torque to bedetermined. Principles of the double alternation filtering type mayusefully be applied. As for the prestressing chain torquing control, anymeans, either to measure the prestressing torque set in the transmissionsystem or to identify the requirement in angular position of theprestressing motor corresponding respectively to the homokinetism errorsto be overcome and to the additional angle corresponding to theprestressing torque to be established, may be employed.

FIG. 2 shows a general block diagram illustrating, firstly, the mainmotorisation chain 4 organised so as to allow a prestressed adjustmentinput to be introduced into the reduction chain of the control of themain chain 7, and secondly, the principle of generation 8 of thevariable prestressed torque using the estimation of the instantaneoustorque to be set into the main chain 7 and the control 6 principleestablishing the prestressing.

The concept of a back-geared motor having variable prestressing is notspecific to a particular gear reducing principle. It is howevernecessary to introduce local modifications allowing at the least aprestressed input into the double chain of the reduction gear. By way ofan example, we quote the following means to perform variableprestressing:

-   -   epicyclic train power stage boxes with prestressed inputs of the        crown gear with motorisation of the prestressing by hydraulic        jack and pressure servo-control, or electrical micro-jack        controlled in position.    -   double-reduction chain box with at least one output gear whose        axis of rotation has a degree of freedom relative to the box        (radial or circular). The pinion is connected to the box output        by an Oldham coupling type system and the relative position of        its axis in relation to the circular is adjusted by a servo        motorisation in position.    -   a double chain reduction gear one stage of which is made of        slanted toothed gear. The associated pinions have an axial        degree of freedom with respect to the box, this degree of        freedom being used to modify the prestressing in the two chains        by a back-geared motor type system.    -   an oscillating aiming box and circular box adjusting mini-jack.    -   eccentric rollers on a rack.

These embodiments are exemplified as follows.

FIG. 3 shows a perspective view of an embodiment of the prestressingsystem 5 integrated into epicyclic train reduction gears 3 and 4.Between the motor 1 and the load 2, there are, in order, the inputpinion 9 a or 9 b, the epicyclic gear train 3 or 4, and the output wheel10. The epicyclic train 3 or 4 classically comprises an immobile crowngear 11 with inner toothing, and input pinion 12 and three planet gears13 meshing on the inner toothing of the crown gear. According to theinvention, a prestressing is introduced by making the crown gear 11mobile in both directions by means of a slight rotation. By slightrotation, we mean a rotation of around 5°. This rotation is obtainedusing a double rack 14 driven in translation by an electricalprestressing jack 15–17. This jack is of a known type and will not bedescribed in further detail.

FIG. 4 illustrates the classical structure of the epicyclic gear trainwhere we find the input pinion 12 integral with the drive shaft 18(connected to the motor 1) aligned along the drive axis XX′, thesatellite gears 13 integral with the output shaft 19, also aligned alongaxis XX′. Classically, the satellites 13 mesh on the inner toothing ofthe crown gear 11. This crown 11 is classically immobile with respect toits support and is made mobile in the invention so as to introduce thevariable prestressing as explained previously.

FIG. 5 shows another embodiment of the variable prestressing means. Itcorresponds to a back-geared motor with a double reduction chain betweena motor 1 and the load 2. Each of the two reduction gears mesh on theinner wheel 10 respectively via pinions 21 and 22. Pinion 21, isangularly connected to the box output axis by a coupling 20 of an Oldhamtype. A hinged connecting rod 24 allows the radial displacement of thepinion 21 with respect to the circular 10. The radial displacement orpinion 21 causes a relative angular displacement of pinion 21 withrespect to pinion 22, thereby modifying the prestressing. The jack 23allows this prestressing to be adjusted.

FIG. 6 shows a section of another embodiment of prestressing meansaccording to which the reduction stage is constituted by slant toothedpinions. Thus, the wheel 26 meshes with pinion 28 via its slantedtoothing. The pinion 28 is driven by the motor 29 by pinions 30 and 31.To adjust the prestressing, pinion 28 is caused to translate, using thepositioning jack 32. The displacement of pinion 28 following arrow Fallows it to be more or less meshed in the toothing of wheel 26 therebycausing the prestressing to vary. The figure does not feature the secondreduction chain, which is quite classical.

FIG. 7 shows another embodiment of prestressing means. It comprises aprestressing jack 35 allowing the reduction gear 3 and 4 to be appliedin an adjustable manner against the toothing of the load 2. The figureshows the main motor 36 and the axis 37 of articulation of theback-geared motor. Naturally, the jack 35 is placed between the gearhousing 38 and the support for the load 2.

1. A servo-control unit for a load drive in rotation by motorizationmeans connected to the load by two reduction gears and comprisingsensors to measure their movements, wherein it comprises a prestressingmotoring system arranged between the two reduction gears so as torespectively apply a positive and negative torque to each of thereduction gear.
 2. A servo-control unit according to claim 1, whereinthe prestressing system is able to firstly apply a torque at any pointof the operating field of the reduction gears and secondly introduceinto it a variable prestressing level.
 3. A servo-control unit accordingto claim 2, wherein the prestressing system is a jack driven by a motorand co-operating with a rack or screw-nut system.
 4. A servo-controlunit according to claim 1, wherein the prestressing system comprises apower stage box having an epicyclic gear train.
 5. A servo-control unitaccording to claim 4, wherein it comprises prestressing inputs on thecrowns of the reduction gear with prestressing motorization provided byhydraulic jack and pressure servo-control.
 6. A servo-control unitaccording to claim 4, wherein the prestressing system is a doublereduction chain assembly integrating a jack and at least one inputpinion whose axis of rotation has a degree of freedom with respect tosaid assembly.
 7. A servo-control unit according to claim 6, wherein thepinion is connected to the output of said assembly by an Oldham couplingtype system.
 8. A servo-control unit according to claim 7, wherein itcomprises a connecting rod hinged so as to ensure the radialdisplacement of the pinion.
 9. A servo-control unit according to claim4, wherein one of the reduction gears has a slant toothed gear stage.10. A servo-control unit according to claim 9, wherein the pinion of thestage has an axial degree of freedom with respect to the wheel, saiddegree of freedom being used to modify the prestressing of the twochains by a back-geared motor.
 11. A servo-control unit according toclaim 4, wherein the prestressing means comprise a jack allowing thereduction gears to be applied to the load.
 12. A servo-control unitaccording to claim 1, wherein the prestressing motorization systemincorporates electrical control means.